Hot gas engines in general utilize the approximately ideal relationship between the pressure, volume, and temperature of gases (i.e. PV=nRT) to control conversion of heat to mechanical power. Internal combustion engines in particular take in a given volume of air at ambient temperature, raise the temperature of that air very rapidly by igniting a small quantity of fuel evaporated or dispersed in it, and provide for a certain amount of mechanical expansion of the hot combustion products to recover some pressure-volume work before the hot gases are expelled from the engine.
An ideal engine/transmission system will recover the maximum amount of work from fuel consumes, at any rate dictated by the need for power, using a simple and easily constructed mechanism of minimum weight. Conventional internal combustion engines, based upon the piston-and-bellcrank displacement mechanism in common use since the time of James Watt, fall far short of the ideal performance largely because of a limited number of practical considerations:
Combustion Kinetics: Because fuel droplets or particles evaporate and burn at finite rates, the temperature of exhaust gases must be somewhat less than the value calculated from thermodynamic properties of the fuel and the air/fuel ratio; the discrepancy increases as engine speed is increased because less time is available for completion of combustion within the engine.
Mechanical Constraints: Because the intake and power stroke displacements of a typical piston-and-bellcrank engine are exactly the same, the combustion gases are still at a high pressure at the end of a power stroke. Escape of this high pressure gas when an exhaust valve opens generates considerable noise and forfeits a significant portion of the pressure-volume work which could have been recovered if the power stroke displacement were sufficiently larger than intake displacement to exhaust the hot combustion gases at ambient pressure.
Load Characteristics: Because the displacement of an engine is specified primarily on the basis of its peak power requirement, any effective means of averaging the load requirements will permit use of a smaller engine, with corresponding reductions in weight and fuel consumption. Furthermore, complex multicylinder engines with flywheels of relatively low rotational inertia are commonly used whenever the engine is expected to change speed rapidly under load, whereas a much simpler and more efficient one-cylinder engine of comparable displacement driving a large heavy flywheel at constant speed could be used to supply the same amount of power if speed variations could be accomplished by a transmission instead of by the engine. Load averaging depends on the ability of a power transmission system to store or withdraw energy from a storage device (e.g. battery, compressed air tank, flywheel, etc.) while continuing to transmit the precise amount of power needed to satisfy the load requirements.
Material Limitations: Because common metallic materials require cooling and lubrication to prevent galling and oxidation of sliding surfaces in an engine, approximately 30% of available combustion heat must be wasted through conduction to a cooling system simply to prevent a conventional engine from destroying itself.